1. Field of the Invention
The present invention relates to a semiconductor memory device and, more specifically, a dynamic random access memory (DRAM) switchable to a binary (2-value) memory or a multi-value memory.
2. Description of the Background Art
Generally, 1 bit data signal is written to a memory cell by charging a memory cell capacitor to VCC (power supply voltage) or GND (ground voltage). A memory allowing writing of two different values in one memory cell is referred to as a binary memory.
A memory cell allowing writing of three or more different values in one memory cell is referred to as a multi-value memory. A 4-value memory, for example, allows writing of a data signal of 2 bits in a memory cell by charging the memory cell capacitor to VCC, (2/3).multidot.VCC, (1/3).multidot.VCC or to the GND. Therefore, storage capacity of the multi-value memory can be remarkably increased as compared with a binary memory, while control circuitry for writing and reading becomes complicated.
In a notebook type personal computer, for example, it is preferred that the main memory has large storage capacity while an application program is active, whereas it is preferred that the main memory has small power consumption in a suspended state.
The storage capacity, however, is not sufficient, as the binary memory is generally used for the main memory. In the binary memory, capacitor capacitance of the memory cell is made small to increase storage capacity which results in shorter refresh period and greater power consumption.
Though it is possible to use a multi-value memory for the main memory, there is a disadvantage that the multi-value memory has too large a storage capacity in the suspended state, resulting in large power consumption.